The present invention relates to a step automatic transmission employed as a transmission for vehicle.
Japanese Patent Application Publication No. 2004-176765 or U.S. Pat. No. 6,648,791 (FIG. 23 and FIG. 25) discloses an automatic transmission adapted to achieve seven forward speed-ratios by using three planetary gear sets. In the automatic transmission disclosed in the above Japanese Patent Application, the seven forward speed-ratios are obtained by using six friction elements and three single-pinion-type planetary gear sets. The single-pinion-type planetary gear set has an advantage in gear noise and also an advantage in durability because of the non necessity for reducing a diameter of pinion gear. Moreover similarly, in the automatic transmission disclosed in the above United States patent, the seven forward speed-ratios are obtained by using five friction elements and three single-pinion-type planetary gear sets.